There's no such option in avast!.The boot-time scanner is meant for cases when you suspect something is wrong; it takes quite a lot of time, so running it on every boot would slow down the usual work unacceptably.

Apart from what has already been said, avast does run scans whilst your system is booting, check the Scanned count: in the Standard Shield provided details and you will see that it does scan at boot but not, the complete system as in a pre windows boot-time scan. A boot-time scan takes some considerable time as not to be practical for every boot.

Is there something specific that you are trying to achieve in wanting to do this ?

Hi, i just read this thread and i'm looking for something similar. Basically i've been infected with Sdbot.gen and my system is riddled right now. I installed avast home edition and rebooted where it began to do a complete system scan but i esc'd as i was in the middle of something, thinking that Avast would do a complete system scan the next time i rebooted but it hasn't! - which leads me to believe that it only does it the first time its installed? - my question is: is there anyway of getting AVAST to do this complete scan again - like over night - was looking at the system settings and there doesn't seem to be an option to do this?

The boot-time scan is a one shot deal, to be scheduled for the next boot when required.

The boot-time scan isn't designed for always on as it would infuriate users waiting the completion of the scan, but as a one time selection to overcome issues where the regular scan can't deal with it. For instance, files in use, files in locations protected by windows, etc. so there is a specific purpose for running the boot-time scan, you can also change what it scans also the default action it takes relevant to the task/problem.